2 years ago

How to distinguish between polar and non-polar molecules/compound? For instance, one that you have not studied before but have to know if its polar or non-polar. Can you distinguish it from the basic structure of the molecule?
For instance, Bromine Br_2; at first I thought it was polar but then its not non-polar..so how can you tell?

You can instantly tell if a molecule is polar if you look at the electronegativity differences between two differing atoms. The larger the difference between electronegativity values, the more polar the molecule is.

halcyon is right. Do you have an electronegativity chart? Some are different from others. IS there one in your text book? If not I can find you one, but its better to use what your teacher supplies you with.

But a non-polar molecule would have an electrongativity differences as well; i think.
and electronegativity charts are not provided in exams here
is it possible to distinguish it from the structure of the molecule?

Not sure about how to find the answer from structure.
"But a non-polar molecule would have an electrongativity differences as well; i think." You are right. The way you determine the status is to see to which degree of difference is apparent.

It's entirely about the structure, like you were thinking! Basically symmetry means non-polar and asymmetry means polar. So Br2 has two atoms with the same EN; you can imagine their electron withdrawing strength canceling out one another's pull so that there's no net pull in the structure and the whole thing is non-polar.
In contrast, something like CO would have a net polarity because even if you can't remember what the EN of C of O is, you still know that one half of that molecule is going to pull the electrons a little more than the other. That gives the molecule a net polarity.
It helps if you just draw a little arrow pointing in the direction of the Electron withdrawing activity. Then you can see if there's a net pull or if there are arrows that cancel one another out.

yes, you have to look at their electronegativity (how much they attract electrons). If there is high electronegativity differences it's most likely a polar bond (e.g. NaCl). But if they got close to no difference (e.g. C-H), then it's a non-polar bond.

on top of that, you have to use the 1st rule i mentioned above, a polar bond may be cancelled out by another polar bond on the exact opposite side...e.g. CCl4, even tho the C-Cl bonds are polar, the exact conformation of the molecule allows all the polarity to cancel out to 0